Thyroid Binding Globulin (TBG) a single polypeptide chain of 415 amino acids which has a molecular weight of 63,000 daltons, as it is the major thyroid hormone transport protein in human serum with the highest association constant (Ka), with a Ka for T4 of 1010 M and for T3 of 109 M (Tetuya Shirotani et al., 1992; DeRuiter, J., 2001). It belongs to the serpin family in genomics and the specific gene for its transcription is located on the sex chromosome X at the locus Xq22.3, but the protein has no inhibitory function like many other members of the serpin family (NCBI, 2016). It is very stable although it loses its binding properties by denaturation at temperatures over 55°C and pH below 4. “Optimal binding activity requires the presence of the L-alanine side chain, an unsubstituted 4′-hydroxyl group, a diphenyl ether bridge, and halogen (I or Br) constituents at the 3,5,3′ and 5′ positions” (Samuel Refetoff, M.D., 2015).